Tìm `x` `x^2-9x=-8` 1 câu thì 10đ đc rồi

`x^2 -9x = -8`

`<=> x^2 -9x + 8 =0`

`<=> x^2 - 2 . x . 9/2  + 81/4 -49/4 =0`

`<=> (x-9/2)^2 = 49/4`

`<=>`   \(\left[ \begin{array}{l}x -\dfrac{9}{2} = \dfrac{7}{2} \\x -\dfrac{9}{2} = \dfrac{-7}{2} \end{array} \right.\)  

`<=>` \(\left[ \begin{array}{l}x=8\\x=1\end{array} \right.\) 

Vậy `S = {8; 1}`